Signal recording apparatus



July 5, 1949. H. E. ROYS 2,475,200

SIGNAL RECORDING APPARATUS Filed June 28, 1945 ZIlII/I/I/IA m "E? Eur/1111114.

l 43 INVENTOR V 59 BY Q ra flMPL/F/Ez ATTORNEY Patented July 5, 1949 SIGNALv RECORDING APPARATUS Henry E. Roys,

Delaware Indianapolis; Ind., assignor to Radio Corporationof America,

a corporation of Application June 28, 1945, Serial 120,601,963 '5 Claims. (Cl. 179-4004) This inventionrelates toelectrical sound recording apparatus, and more especially to a recordcutting system which utilizes audio frequency feedback.

In electrical sound recording. systems employing. an audio frequencyamplifier which. feeds acutter driving c011,. it has. been proposed, heretofore, to make use ofdegenerative or negative audio frequency feedback to; the amplifier. Generally, in these prior systems, an audio frequency voltage is derived fromvibration of the cutter armature andthis voltage is fedback to the amplifier in a degenerative sense so. as to minimize harmonics. Other advantages are. also. securedby means. of the degenerative feedback arrangement. Such systems-are. generally, illustrated in the prior art by U. S; Patent No. 2,161,489 grantedto Leonard Vieth, on.June,6,'1939, and U. S.'Patent No. 2,162,986 granted to Charles F. Weibusch on June 20,1939. In, these various systems, it is important that there be a minmum of interaction between thereactance element which drives the cutterand thereactance element which generates the audio frequency feedback voltage. Various devices have been. utilized in the past to prevent interaction between these reactive elements. It is also important, insystems. of this type, to insure mechanical synchronization between the driving reactance element and the feedback reactance element 50 that the audio frequency feedback voltage is accurate in representation of the audio frequency current output of the microphone which feeds thedriving amplifier.

The primary object of. my present invention is to provide an improved electrical sound recording apparatus which will have these and other th sirable characteristics.

Another object of my present invention is. to provide an improved system of the type referred to above, my improvement consisting essentially of ,at least a. partIy. hollow supporting shaft for the cutter armature; the supporting shaft housing within .its interior, in somewhat spaced relation to the driving coiLa reactance element which provides the audio frequency feedback voltage.

Another important'object of my present invention is toprovide a record'cutting system em ploying'a cutter armature having diametrically opposed extensions which-are surrounded by the driving coils-,"thearmature being carried by a supporting shaft one end of which is hollow" for housinga feedback coil which is located in the fieldof a, permanent magnet whereby vibration of the armature causes generation of audio fre- 2 quency voltages in the feedback coil,.the driving coil and feedback coil being sufficiently spaced from each other to prevent any substantial interaction between the two.

A'furtherand also very important object of my present invention is to provide an improved recording system as aforesaid which employs stabilized feedback and in which distortion is thereby reduced to a minimum.

. Still another object ofmy present inventionis to providean improved recording system as above set forth which will have a substantially flat frequenoy response, largely as a result of the fact that a rigid mechanicalsystem with but a single resonant frequency is employed in contradistinction to the compliantly coupled, multi-section mechanical systems generally employed in the prior art and inwhich each section is resonant at a'difierent frequency.

An additional object of my present invention is toprovide an improved recordin system as above set forth the frequency response and. sensitivity of which will not change materially with variations in temperature.

It, is also an object of my present invention to provide an improved recording system as set forth above of which the, frequency response will be independent of the load applied to the cutting or recording stylus.

' Still other objects of this invention are to improve generally the efficiency and reliabilit of record cutter systems of the type specified above, and more especially toprovide such systems which are economical in cost, durable in construction, and highly efficient in use.

The novel features which I believe to be characteristic of my inventionare set forth in .particularity in. the appended claims. The invention itself, however, both as to its organization and method of operation,.as well as additional objectsand advantages thereof, will best be understood from the following description, when read in connection with. the the accompanying drawing, in which:

Figure 1 isapartly sectional and partlydiagrammatic view showin a circuit organization according toone formof. my. invention,

Figure 2 is a detailed view of the pick-up coil andmounting therefor of Fig. 1,

Figure 3 is an .end elevation of a feedback unit according to my present invention employing a short-circuiting shield,

.Figure 4 is anenlarged, fragmentary, sectional, view showing a. different form of mounting for the feedback coil,

Figure 5 is a fragmentary, perspective view of another modification of my invention in which a different form of feedback unit is employed, and

Figure 6 is an end elevation of a feedback unit similar to that shown in Figure 5 and supplied with a short-circuiting shield and with bucking coils around the pole pieces.

Referring more particularly to the drawing, wherein conventional operations of the system are schematically represented since they are well known to those skilled in the art of cutting records, there is shown, in Fig. 1, a turntable i of conventional construction which carries a record blank 2 on the upper surface of which the impressions corresponding to desired sounds are to be recorded by engraving or cutting. Since those skilled in the art of cutting records are fully familiar with the details of mountin the record on the turntable and proper centering of the record, it is not believed necessary to describe such details any further.

A microphone 3, which may be of any well known and desired form, picks up the sound to be recorded and feeds to an audio frequency amplifier 4 the audio frequency currents which it generates. The amplifier 4 may be of any well known construction. It is desirable to use an amplifier which has a wide band characteristic. More specifically, it is desirable to use an audio frequency amplifier of several cascaded stages and which has substantially zero phase shift up to 30,000 cycles per second or over. Here, again, such an amplifier is well known to those skilled in the art and therefore need not be described in further detail.

The amplified output of the amplifier 4 is fed to a pair of fixed armature driving coils 5 and 6. These coils are disposed, respectively, about a pair of somewhat spear-shaped arms or sections l and 8 which extend in diametrically opposed relation from the hub or sleeve section 9 of a laminated armature Hi. The laminations of the armature may be held together by cement, rivets, or any other suitable means. It will be noticed that the coils 5 and 5, as shown in Fig. 1, are connected in series between the output terminals of the amplifier 4. However, the coils 5 and 5 may be connected in parallel relation depending upon the electrical impedance desired. A stylus bar H, which may have the same shape, generally, as the armature I0, is secured to the armature and carries a cutting or engraving stylus [2 for coaction with the record blank 2 in well known manner.

A permanent magnet l3, which may be of the usual U-shape, provides a fixed, magnetic field for the armature Ill. The magnet I3 is provided with two sets of opposed pairs of pole pieces adjacent the tapered ends of the armature sections l and 8, only the North poles I 4 and 15 being shown in the drawing, since this arrangement is quite well known. Thus, the tapered end of the armature section 1 is disposed between one cooperating pair of north and south pole pieces, and the tapered end of the armature section 8 is likewise disposed between the other pair of cooperating north and south pole pieces.

The armature is secured, through its hub Q, to a horizontal shaft l6 of light-weight, nonmagnetic material (such as aluminum, magnesium, suitable alloys of either of these metals, or the like), the shaft [6 being substantially rigid torsionally and being hollow at the end remote from the armature I 0 for a purpose to be presently set forth. For rotatably mounting the shaft IE, it may be provided with shaft extensions IT and 18 (the latter also hollow) of reduced diameter, the extensions I7 and I8 being supported in suitable bearings (not shown) in well known manner. As the audio frequency signal currents flow through the armature driving coils 5 and 6, the varying magnetic fields set up thereby will react with the permanent magnetic field through the laminated armature supplied by the magnet 53, and as a result, the armature Ii) will vibrate at the signal frequencies to effect vibration of the stylus bar H and the cutter 12. Thus, the cutter I2 will cut a track or groove in the blank 2 which will correspond to the audio frequency signals picked up by the microphone 3.

In accordance with my invention, audio frequency feed back voltage is fed to the input of the amplifier 4 so as to eliminate harmonics and secure other well known advantages. For this purpose, there is mounted within the hollow or tubular portion 16a of the armature shaft IS a pick-up or feedback coil 20 wound on a suitable form 2!, the coil being arranged with its axis normal to that of the driving coils 5 and 6 to minimize coupling therebetween. The output of the feedback coil 20 is fed back to the input of the amplifier 4 by a pair of leads 22 which are brought out through the hollow shaft extension !8. In general, the voltage fed back to the amplifier 4 is degenerative or negative voltage. However, in some cases, regenerative or positive feedback voltage may be employed to advantage, as explained by H. S. Black in his article Stabilized feedback amplifiers, Bell System Technical Journal, vol. 13, Jan. 1934, page 9, paragraphs 1 and 2.

Cooperating with the coil 20 is a permanent magnet 23 so disposed that the coil 20 lies in the air gap between its pole tips 24. The magnet 23 supplies a fixed magnetic flux through the coil 20 and constitutes therewith an electrodynamic system of well known form. As the rigid shaft l6 rotates in response to the signal energy fed through the driving coils 5 and 6, it will simultaneously similarly rotate the feedback coil about its (the shafts) axis to synchronously vibrate the coil 20. The turns of the coil 20 will then more or less cut the magnetic lines of force in the field supplied by the magnet 23 and, as a result, voltages will be induced in the coil 2d which are of audio frequency and which are accurate counterparts of the audio frequency currents flowing in the driving coils 5 and 6. By feeding these audio frequency voltages back to the amplifier 4, the advantages of my present invention are realized.

The feedback coil 29 and the driving coils 5 and 6 should be spaced from each other a distance such that there .can be very little interaction between these reactive elements. Furthermore, since the pickup coil 20 is housed within the interior of the tubular section of shaft Hi, this also provides a degree of shielding for the coil 20. In addition, the entire tubular section Isa (or at least that portion of it which is occupied by the coil 20) and the pole piece tips 24 may be completely enclosed in a copper shield 25 for greater shielding, as shown in Fig. 3. The feedback voltage generated across the coil 2?) must of necessity be an accurate representation of the velocity of the shaft !6 and the armature coupled thereto. Since the shaft [6 is positively locked to the armature l0 and also to the coil form 2|, and since the shaft I6 is rigid 5: torsionally, thereswill bearigidmechanical coupling between the coil 20 and the armature H1. This is" one-f the advantages'of my present-invention, namely, the fact that--a--substantially perfect mechanical rcoupling "is secured between the armature I it and the feedback coil 21].

,In, the form of my inventi'omshown in the: tubular shaft section lfiwis provide'd with aligned openings 38' andthe" feedback coil 2c is wound "directly on "the shaft l Bithrou'gh these openings. As in thecase of-'-Fig.- l, theaxis'of the feedback coil 20 is normal to that of the driving coils and 6 to minimize magnetic coupling therebetween. Also, since the shaft H5 is of nonmagnetic material, this will further tend to minimize the coupling between the feedback and the driving coils.

In the form of my invention shown in Fig. 5, the armature shaft portion I600 is formed with a diametrical slot lGb in which is placed a magnetic armature 40. The feedback coil 20 is wound on the armature All which is rigidly secured to the shaft l6, and the armature 4i! cooperates with a pair of substantially semi-circular pole pieces 4| which embrace the shaft portion Hid and which are supplied with flux by a permanent magnet 42. Since the armature Hi, the shaft l6 and the armature 40 are rigidly coupled together for vibration as a unit, it will be apparent that vibration of this unit will cause voltages at signal frequencies to be generated in the feedback coil 20. If desired, a copper shield may be placed around the feedback coil, its armature 40 and the field structure 4|, 42 to minimize coupling between the coil 20 and the coils 5 and 6. If such copper shielding or the shielding of Fig. 2 (which is more effective at the high frequencies than at the low frequencies) is insufficient, it may be found advantageous to add bucking coils 43 (Figure 6) around the pole pieces of the feedback unit, and perhaps also around the shaft IE. Such bucking coils should, of course, have the proper number of turns connected in series with and in correct phase relation with respect to feedback coil 28 so as to oppose and cancel out any voltage generated in the latter coil due to coupling between the coils 5 and 6 and the coil 25%. This method of cancellation is quite well known and is disclosed in the above identified Vieth patent.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many other modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

I claim as my invention:

1. In signal recording apparatus, the combination of an oscillatory system comprising a rotary shaft which is substantially rigid torsionally, a magnetic armature rigidly secured to said shaft, and a feed-back coil also rigidly secured to said shaft, said shaft, armature and coil being rotatable as a substantially rigid unit on the axis of said shaft, a driving coil associated with said armature, means including an amplifier for feeding signal current to said driving coil whereby to effect rotary oscillation of said unit at signal frequencies, means for inducing voltages in said feed-back coil at signal frequencies upon oscillation thereof at said frequencies, and means for feeding back to the input of said amplifier the output voltages of said feed-back coil, said feed- 6 back 'coil.-being.= so fdisposed relative :t0iESaidI driV-'- ing: coililthatz' the axesoisaid: coils ..are-: substantially' normal to .each other, whereby saidrcoils are insubstantially noneinductive'relation to each 4 shaft,=' saidshaft;- armature and coil being rotatable:asa substantially rigidunit on the axis of saidshaft, adriving coil associated with said armature, means including an amplifier for feeding signal current to said driving coil whereby to effect rotary oscillation of said unit at signal frequencies, means for inducing voltages in said feed-back coil at signal frequencies upon oscillation thereof at said frequencies, and means for feeding back to the input of said amplifier the output voltages of said feed-back coil, said shaft having a hollow portion, and said feed-back coil being mounted within said hollow portion.

3. In signal recording apparatus, the combination of an oscillatory system comprising a rotary shaft which is substantially rigid torsionally, a magnetic armature rigidly secured to said shaft, and a feed-back coil also rigidly secured to said shaft, said shaft, armature and coil being rotatable as a substantially rigid unit on the axis of said shaft, a driving coil associated with said armature, means including an amplifier for feeding signal current to said driving coil whereby to effect rotary oscillation of said unit at signal frequencies, means for inducing voltages in said feed-back coil at signal frequencies upon oscillation thereof at said frequencies, and means for feeding back to the input of said amplifier the output voltages of said feed-back coil, said shaft having a hollow portion with said feed-back coil mounted therein, and said armature being secured to said shaft in spaced relation to said feedback coil longitudinally along said shaft.

4. In signal recording apparatus, the combination of an oscillatory system comprising a rotary shaft which is substantially rigid torsionally, a magnetic armature rigidly secured to said shaft, and a feed-back coil also rigidly secured to said shaft, said shaft, armature and coil being rotatable as a substantially rigid unit 0n the axis of said shaft, a driving coil associated with said armature, means including an amplifier for feeding signal current to said driving coil whereby to effect rotary oscillation of said unit at signal frequencies, means for inducing voltages in said feed-back coil at signal frequencies upon oscillation thereof at said frequencies, and means for feeding back to the input of said amplifier the output voltages of said feed-back coil, said shaft having a hollow portion adjacent one end thereof with said feed-back coil mounted therein, and said armature being secured to said shaft adjacent its other end.

5. In signal recording apparatus, the combination of an oscillatory system comprising a rotary shaft which is substantially rigid torsionally, amagnetic armature rigidly secured to said shaft, and a feed-back coil also rigidly secured to said shaft, said shaft, armature and coil being rotatable as a substantially rigid unit on the axis of said shaft, 9, driving coil associated with said armature, means including an amplifier for feeding signal current to said driving coil whereby to effect rotary oscillation of said unit at signal frequencies, means for inducing voltages in said 8 feed-back coil at signal frequencies upon oscillation thereof at said frequencies, and means for REFERENCES CITED feeding back to the input of said amplifier the The ow g references are of record in the output voltages of said feed-back coil, 8, bucking fi this patent! coil in said unit and rotatable therewith, said UNITED STATES PATENTS bucking 0011 being serially connected with sa1d feed-back coil and therewith being so inductively Number Name Date coupled and related to said driving coil as to gen- 1,535,533 M d DI. 5 erate counter voltages and balance out from the 2,161,489 Vleth June 1939 circuit of said feed-back coil Voltages induced 10 2,229,326 ll r Jan.21, 1941 therein as a result of any stray inductive coupling 2,351,353 McCarty June 3, 1944 between said driving coil and said feed-back coil.

HENRY E. ROYS. 

